Many battery-powered consumer electronic devices (“host devices”) can include attachable options (“attachments”) that themselves can have batteries. An example might be a personal digital assistant (PDA) or camera/camcorder to which can be attached a wireless modem card, for transmitting data to and from the PDA/camera/camcorder. A separate battery on the attachment can be required under some circumstances because design limitations on the amount of current flow through the interface between host and attachment limit the amount of current that can be supplied to the attachment from the host battery.
The present invention understands that for size and cost purposes, it is desirable to minimize the size of the battery particularly on the attachment, with the electrical load of the combined system being shared between the battery in the attachment and the host battery. As an example, a code division multiple access (CDMA) wireless modem card attachment that is designed to meet the Compact Flash Interface specification can require more than 800 mA at peak transmit output power, but the compact flash interface allows the host battery to supply no more than 500 mA across the host-attachment interface. For this reason, a separate battery must be provided on the card, and it can share the power load with the host battery.
The present invention critically observes, however, that it can be difficult to join battery power supplies by simply tying them together. Even though the batteries may be the same type, the voltages can be different based on their relative states of discharge and relative temperatures. Under circumstances of such environmental imbalance, charge from one battery can immediately begin to flow into the other battery, and may flow back across the interface, which might be forbidden by the design specification. On the other hand, if the batteries are separated from each other by, e.g., supplying separate parts of the card from each battery, it may be difficult to find a good division where the peak currents required of both batteries satisfies design limits.
Moreover, different circuits may have different and hard to predict voltage needs. With particular respect to a CDMA card attachment, for example, its power amplifier exhibits a very non-linear, roughly exponential, requirement for current versus output power. Consequently, due to this wide variation of current demand from the CDMA modem, mostly attributable to the power amplifier, it is not possible to predict the amount of current the power amplifier will require relative to the rest of the CDMA modem card at any one time. The result is that a larger than needed attachment battery might be required, with its capacity most of the time going to waste. As understood by the present invention, while it is desirable to minimize the size of the attachment battery, it is also desirable to achieve a balance whereby one battery does not always exhaust itself before the other battery.
The present invention has considered the above-noted problems and has provided the solutions below to one or more of them.